The present invention relates to an image-pickup apparatus such as a video camera and a digital still camera, and more particularly, to focus control in such an image-pickup apparatus.
In autofocus (AF) control for image-pickup apparatuses such as video cameras, a TV-AF method is primarily used which includes using an image-pickup element to produce a video signal, producing an AF evaluation value signal corresponding to the sharpness (contrast state) of the video signal (picked-up image), and searching for the position of a focus lens where the AF evaluation value signal is at the maximum.
Another AF method is an external ranging method (external phase difference detection method) in which a ranging (distance-measuring) sensor is provided independently of an image-pickup lens to detect the distance from an object, an in-focus position of a focus lens is calculated on the basis of the detected distance, and the focus lens is moved to the in-focus position.
The external ranging method involves splitting a luminous flux from an object into two which in turn are received by a pair of light-receiving element arrays (line sensors). A difference between the images formed on the pair of line sensors, that is, a phase difference, is detected and used to determine the object distance with triangulation, and the focus lens is moved to the in-focus position for that object distance.
To take advantage of the high accuracy of focusing in the TV-AF method and the fast focusing in the external ranging method, a hybrid AF method has been developed by combining these AF methods and disclosed in Japanese Patent Laid-Open No. 2005-84426, for example. In the hybrid AF method disclosed in Japanese Patent Laid-Open No. 2005-84426, focusing is obtained with the phase difference detection method when it is determined that the TV-AF method cannot achieve the focusing due to low luminance or low contrast of an object.
Japanese Patent Laid-Open No. 5(1993)-64056 has disclosed another hybrid AF method in which a focus lens is driven to near an in-focus position in a TTL phase difference detection method and then the focus lens is driven with higher accuracy to the in-focus position in the TV-AF method. Japanese Patent Laid-Open No. 2005-234325 has disclosed another hybrid AF method in which either the TV-AF method or the external ranging phase difference detection method is selected in performing focus control based on the amounts of change of signals produced in the two methods.
In the hybrid AF using the TV-AF method and the external ranging method in combination, correct information on an object distance cannot be provided if an obstruction which is not an object is present between a ranging sensor and the object. The ranging sensor is typically placed close to an image-pickup lens to minimize parallax between an image-pickup field for picking up images through the image-pickup lens and a view field for detection (detection view field) in the ranging sensor. In this case, a hand of a user, for example, is often put between the ranging sensor and an object to block an entrance of light from the object on the ranging sensor. The blocking of the light from the object to the ranging sensor leads to a longer time taken to achieve focusing or an inability to achieve focusing, so that it is impossible to make the most of the advantage of the fast focusing which results from the external ranging method used in combination with the TV-AF method.
To solve the problem, Japanese Patent Laid-Open No. 2005-241805 has disclosed an image-pickup apparatus in which focus control is performed with the TV-AF method instead of the external ranging method when a pair of light-receiving element arrays in a ranging sensor provide outputs which differ by a threshold amount or more.
The method disclosed in Japanese Patent Laid-Open No. 2005-241805 is effective when only one of the paired light-receiving element arrays in the ranging sensor is covered with an obstruction such as a hand of a user. However, if both of the paired light-receiving element arrays are covered similarly with the hand of the user, for example, the difference between the outputs from the pair of light-receiving element arrays is smaller than the threshold and thus focus control may be performed with the external ranging method. In other words, the focus control may be performed on the basis of the incorrect object distance information, thereby resulting in a longer time taken to obtain an in-focus state or an inability to obtain the in-focus state.